• Merry Havens

Math Trick for Multiplying numbers 2 apart

Updated: Oct 25, 2019

For example, if you are multiplying 19 x 21, do this:

square the middle number (multiply it by itself) 20 x 20 = 400

subtract 1, there's your answer: 399


Practice: 29 x 31

square the middle number (30 x 30) = 900

subtract 1, there's your answer: 899


It works with single digits, try 5 x 7


try 4 x 6


but it's really handy for the bigger numbers.


Try 99 x 101


100 x 100 is 10,000, so the answer is 9,999


OK, ready for the slightly more challenging ones?

Try 23 x 25.....try it on your own before scrolling to the answer....





24 x 24 is (24 x 20 ( 480) + 4 x 24 (96) = 576) - 1 = 575

You might be (rightfully) asking "How is this easier than just multiplying 23 x 25?"

A: It's just another tool in your toolbox of strategies, giving you more options to choose from. You'll see it's easier to use this method if the middle number is easy to square.

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